A variety of biochemical and genetic techniques will be used to study the behavior of retroviruses, the oncogenes they carry, oncogenes they activate by insertion mutation, and other putative oncogenes. Emphasis will be placed upon the mechanism of proviral insertion into host chromosomes, modes of regulation of retroviral gene expression, the relationship between proviruses and the transposable elements of flies, the use of retroviruses as genetic vectors, the structure and function of viral and cellular src genes and of several proto-oncogenes (c-myc, int-l, c-Ha-ras) activated during tumorigenesis by proviral insertion mutations, and the identification of novel oncogenes.